[Pw_forum] Functionals, pseudopotentials, etc. for H2O ice

[Konstantin Kudin]

--- Axel Kohlmeyer <axel.kohlmeyer at theochem.ruhr-uni-bochum.de> wrote:

> your main problem here should be the representation of the dispersion
> interaction contribution at all, which is not very good in plain DFT.
> there have been serveral suggestions on how to overcome this
> limitation: 
> using an additional and empirically fitted attractive lennard-jones 
> potential, modifying the nonlocal component of pseudopotentials using
> 
> a pertubation theory based optimization scheme, and most rigorously 
> using hybrid functionals, that include hartree-fock or
> exact-exchange.

 Well, it is not that useful to lump all types of weak interactions
into dispersion that DFT supposedly does not describe. As a matter of
fact, hydrogen bonds do have a rather strong Coulomb and density
overlap component, so plain DFT description of that can be quite
reasonable. Such bonds are quite different from those in something like
He dimer, where the lowest level of physically proper description is
some kind of perturbation theory like MP2. Hybrid functionals do work
somewhat better, however, the expense is much greater as well.

 I think there was a recent paper assessing a new meta-GGA functional
TPSS for hydrogen bonds, and it probably also contains data for other
functionals as well (Staroverov, Scuseria, JCP, 2004 [or 2003]). If I
recall correctly they concluded that TPSS could be as good as hybrids
for hydrogen bonds while being cheaper computationally.

 Kostya


		
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